Conventionally, there are well known the linear motion guide unit in which a slider fits over or conforms to a track rail rectangular in transverse section for sliding movement relatively to the track rail by virtue of more than one rolling element. The linear motion guide units of the type recited earlier have been employed extensively in machinery in various fields such as assembling machines, semiconductor manufacturing apparatus, inspection/measurement instruments, and so on.
Meanwhile, modern linear motion guide units have been especially required more shrinking in construction even with ensuring high precision and accompanying reduced maintenance for lubrication. The linear motion guide units have been further needed to make smooth reciprocating motion to live up to high sliding velocity and high cyclic operation of the machines. With the linear motion guide units used in advanced machines, the slider is needed to travel with much smoothness even on the track rail lying in a vertical position or posture other than the common horizontal posture. The slider in the linear motion guide unit in general has forward and aft end caps made therein with turnaround passages, which are arranged to take a detour around any upper edges of the track rail to introduce the rolling elements from a load-carrying race on both sides of the track rail into a return passage lying biased more sidewise inwards than the race so as to make the slider small in widthwise dimension.
Advanced machines of the sort as stated earlier, nevertheless, look for linear motion guide units able to make much higher sliding motion and higher cyclic operation, even with accompanying more reduced maintenance for lubrication.
In the commonly assigned Japanese Patent Laid-Open No. S60-256619, there is disclosed a linear motion guide unit in which a slider is especially made reduced in widthwise measurement thereof. With this prior linear motion guide unit recited now, the track rail is made on both sides thereof with raceway groves extending lengthwise of the track rail, one on each side, while the slider is composed of a carriage and forward and aft end caps, the carriage being provided therein with return passages in a way a return passage lying biased more sidewise inwards than the rolling elements in the load-carrying race at a height over the rolling elements, and the end caps being made on their inward surfaces with turnaround passages in a widthwise symmetrical fashion. With the linear motion guide unit constructed as stated earlier, the rolling elements rolling through the load-carrying area gradually shift to a sidewise outward ingress of the associated turnaround, then following a U-shaped track to reach the associated return passage out of the turnaround passage.
Another linear motion guide unit having a slider reduced in widthwise measurement is disclosed in another commonly assigned Japanese Patent Laid-Open No. H01-126422. With this prior linear motion guide unit, the turnaround passage in an end cap is made in the form of a curved groove lying perpendicular to an axis extending in a sliding direction of a major carriage in the slider. The curved groove is cut in the end cap in a way the radius of curvature thereof is constant and also the center thereof lies on a straight line extending through the centers of sequential rolling elements in the load-carrying race and in parallel with the lengthwise side of the track rail on which the raceway groove is cut deep. The return passage is placed in the carriage at a level as high as the highest point of the curved geometry of the turnaround passage. Confronting surfaces of the track rail and the carriage are made into inclines on which the raceway grooves are cut to define the load race between them. As an alternative version, there is disclosed another linear motion guide unit in which the raceway groove to form the load race is cut in the perpendicular side on the track rail and the turnaround passage is made in the curved geometry whose center is placed on the perpendicular line passing through the center of the rolling element and whose radius of curvature is constant. The return passage is made in the carriage at a level as high as the highest point of the curved geometry of the turnaround passage in such a way lying directly over the rolling elements in the load-carrying area.
With the linear motion guide unit made slim or narrow in widthwise measurement as stated earlier, the turnaround passage in the end cap is made to have a curve whose radius of curvature (R) is constant whereas the return passage has to be placed at a level as high as the highest point of the turnaround passage. This prior construction needs to make the track rail trapezoidal in transverse section. Otherwise, the track rail made to have perpendicular sides requires providing the return passage lying directly over the rolling elements in the load-carrying area.
A further another linear motion guide unit is disclosed in Japanese Utility Model Registration No. 3008835, in which the combination of a slider major body and end caps is guided along a linear rail with less noise by virtue of successive balls rolling through between the rail and the slider major body. With the prior linear motion guide unit recited now, a non-loaded return passage is made in the slider major body over the load-carrying raceway groove while a pair of U-shaped turnaround grooves is made inside the end cap. Each U-shaped turnaround groove is made semicircular in transverse section and made up of a curved area and a linear area, which are allowed to receive five balls therein. The turnaround groove completes the circulating circuit in a way that the balls enter the U-shaped turnaround groove out of the load race, followed by making a turn up to 180 degrees along the turnaround groove, and reaching a central location in the end cap to roll into the associated non-loaded return passage. Moreover, the end cap fits along the U-shaped turnaround groove thereof over a cap plate that has grooves semicircular in transverse section thereof to cooperate with their associated U-shaped turnaround grooves in the end cap. The semicircular grooves in the end plate have inverse angles of circular contour at their semicircular edges and their central openings.
The linear motion guide unit discussed now, although somewhat helpful in smooth circulation of the balls, would pose a problem that the slider itself is apt to get wobbling or bumpy in sliding travel because the turnaround groove made up of the semicircular area and the linear area comes to have many changes or bents in the turnaround passage, which might thwart smooth rolling of the balls. With the U-shaped turnaround groove in which semicircular grooves joined together, moreover, the trace of the centers of the rolling balls comes to fall in line with the joint line of the semicircular grooves, so that the balls cannot be guided with steadiness. The inverse angles of circular contour in the prior linear motion guide unit recited as stated earlier, because of getting mutual fit area of the end cap with the cap plate edgy contour, adversely make the circulating circuit discontinuous or irregular, interfering smooth circulation of the balls.
A further another commonly assigned Japanese Patent Publication No. H06-54129 discloses a rolling-contact bearing for linear motion system, in which rolling elements are allowed to roll through from a linear area into a curved area and vice verse in the circulating circuit, while shifting in a direction along the circulating circuit at some changes or bents.